Millimeter wave ("MMW") amplifiers operate at very high frequencies, 28 Gigahertz and higher. They employ a monolithic microwave integrated circuit device or "MMIC" die or chip as the active element which produces millimeter wave signal amplification. The MMIC chip and its associated circuitry is housed in a container, including a base, a ceramic substrate, and a covering lid, referred to as a module, and together therewith constitutes the MMW amplifier.
The amplifier module includes a waveguide to microstrip transition or coupling, as variously termed, for coupling the microwave energy introduced into the module through a rectangular waveguide in the module's base or lid. It also contains another like transition for coupling the amplified microwave energy out through another rectangular waveguide, also in the module's base or lid.
The transition includes a probe located in the path of the waveguide and a microwave cavity positioned on one side of the probe that forms a short circuit termination for the waveguide located on the other side of that probe. The cavity defines a length of short circuited waveguide of a length of approximately one-quarter wavelength at the middle of the amplifier's frequency range of operation, about one-quarter centimeter (one-tenth of an inch) at 28 GHz. The probe connects to a microstrip that leads to the MMIC amplifier chip.
The foregoing relationship of the transition elements achieves maximum energy coupling between the probe and the waveguide. Selection of cavity size, probe size and positioning for the transition design is accomplished using conventional design criteria available in the technical literature. Essentially such a cavity is a metal walled cavity whose walls are the same size and rectangular shape as that of the input waveguide. Its closed back end wall, the short-circuit, faces the entry to the cavity and the end of the input waveguide. The back wall of the cavity serves as a short circuit to the end of the input waveguide, a short circuit at the back, hence, the denomination of that microwave cavity as a backshort.
The past practice by the assignee of the present invention was to form that microwave cavity as an integral part of the lid, by simply machining out a rectangular shaped hole about four tenths of an inch deep into the underside surface of the lid. The integrally formed cavity was positioned to lay over the waveguide end in the module base when the lid was put in place. A resilient compressible conductive gasket was placed between those edges and the underlying elements to account for any surface unevenness.
Amplifier modules constructed in that way were found to yield inconsistent performance. That is, one amplifier module produced in a production run yielded certain performance characteristics, and the next amplifier module produced in that production run, although containing seemingly identical parts and assembly techniques as the first, obtained significantly different, hence, inconsistent, results. Though straightforward, simple, and direct the foregoing structure necessarily contributed to that inconsistency. Although not visible to the eye, minute physical differences and changes caused significant changes to the electromagnetic properties of the amplifier module.
At a frequency of 28 Ghz, one wavelength measures just under one centimeter in length or slightly less than four-tenths of an inch. Although also physically small in size, unlike lower frequency apparatus, the physical dimensions of the MMIC chip and the associated transition and transmission line components are large relative to the wavelength of the operating frequency. As a consequence a small physical difference of an amplifier element, whether in geometry, size and/or dielectric thickness, can impact the electromagnetic characteristics of the amplifier module.
Although intended to be identical in construction, in the absolute sense each MMW device in a production run might differ in physically minute respects from others within the production run. Should the substrate be too easily compressed, a change in torque of the screws that fasten the lid could change the geometry, and hence the dielectric characteristic of the ceramic substrate, causing a change in performance between one amplifier and the next. Although the physical change is minute in the absolute sense, measured against the wavelength of the frequencies employed, which is only one centimeter at 28 GHz, the difference is significant. That difference results in a change in the coupling characteristic of the transition between the waveguide and the microstrip.
Accordingly, a principal object of the invention is to simplify and more efficiently manufacture microwave millimeter wave amplifiers and like devices.
A further object of the invention is to manufacture millimeter microwave amplifiers that produce consistent operating performance.
An still further object of the invention is to provide a new module construction for millimeter microwave amplifiers that more easily reproduces in quantity microwave amplifiers that are consistent in performance.
And an additional object of the invention is to provide a new and more effective backshort assembly for the waveguide-to-microstrip transition or coupling in a millimeter microwave module.